feat(elbv2): add metrics to INetworkLoadBalancer and IApplicationLoad…

…Balancer (#23853)

By moving the metrics methods to the `INetworkLoadBalancer` and `IApplicationLoadBalancer` interfaces it allows to create these metrics also for LBs that are imported via the `fromXXX` methods.

To create the metrics for LBs requires only the full name of the LB. This attribute is available at the constructs returned by the `fromXXX` methods.

To solve this problem I did:

- Introduce a new interface for each LB type: `INetworkLoadBalancerMetrics`, `IApplicationLoadBalancerMetrics`
- Create a concrete implementation for the new interfaces (1 for each): `NetworkLoadBalancerMetrics` and `ApplicationLoadBalancerMetrics`
- Make each concrete implementation of each Load Balancer to also provide a `metrics` field. The concrete implementations of the load balancers are: `ImportedApplicationLoadBalancer`, `LookedUpApplicationLoadBalancer`, `ApplicationLoadBalancer` (and the same for the NLB classes).

I chose to create a new interface because code can be reused across the 3 concrete implementations of each Load Balancer. I deprecated the `metricXXX()` methods of each load balancer because I think it is cleaner to access metrics through the new `metrics` attribute/interface.

This task is a step in the direction to fix #10850, but I'd like to get feedback and merge this PR first before proceeding into the `TargetGroup` constructs.

PS: I'm learning Typescript, so please, tell me if I'm doing something wrong here.

----

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### Adding new Construct Runtime Dependencies:

* [ ] This PR adds new construct runtime dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-construct-runtime-dependencies)

### New Features

* [x] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
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packages/@aws-cdk/aws-elasticloadbalancingv2/README.md

@@ -604,3 +604,14 @@ const listener = elbv2.NetworkListener.fromLookup(this, 'ALBListener', {
   listenerPort: 12345,
 });
 ```
+
+## Metrics
+
+You may create metrics for each Load Balancer through the `metrics` attribute:
+
+```ts
+declare const alb: elbv2.IApplicationLoadBalancer;
+
+const albMetrics: elbv2.IApplicationLoadBalancerMetrics = alb.metrics;
+const metricConnectionCount: cloudwatch.Metric = albMetrics.activeConnectionCount();
+```